Fuel injection system for an internal combustion engine

ABSTRACT

A fuel injector assembly for an internal combustion engine, including: 
     a delivery chamber ( 5 ) located within the injector assembly ( 1 ); 
     a mass flow rate control means ( 50 ) for controlling the mass flow rate of fuel and compressed gas supplied to the delivery chamber ( 5 ), the mass flow rate being a function of the differential pressure across the mass flow rate control means ( 50 ); and 
     valve means ( 9 ) for selectively communicating the delivery chamber ( 5 ) to the engine to deliver fuel to the engine; 
     wherein when the valve means ( 9 ) is opened, at least compressed gas is caused to flow thereby generating a differential pressure across the mass flow rate control means such that a controlled fuel flow is provided to the engine.

The present invention is directed to a fuel injection system for aninternal combustion engine. In particular, the present invention isapplicable for direct fuel injection into a combustion chamber of anengine and will be described in this application. It is however to beappreciated that the present invention is equally applicable formanifold and other fuel injection applications.

The Applicant is involved in the development of air-assisted fuelinjection systems for use in internal combustion engines. As a primaryfeature, these systems utilise air to entrain and inject a quantity offuel directly into a combustion chamber of the engine. In one particularsystem, such as that described in the Applicants' U.S. Pat. No.4,934,329 , the contents of which are hereby incorporated by reference,a separate fuel injector and delivery valve are provided for eachcombustion chamber, the fuel injector supplying a metered quantity offuel to a delivery chamber of the delivery valve. The fuel is thendelivered to the combustion chamber by opening the delivery valve sothat fuel is entrained and delivered by pressurised gas. Typically, anair compressor supplies the pressurised gas to the delivery chamber.

In a variation of the above system, such as that described in theApplicants' U.S. Pat. No. 5,622,155 , the contents of which are herebyincorporated by reference the pressurised gas is supplied from an airchamber in communication with the delivery chamber. Combustion gasesfrom the combustion chamber are allowed to enter this air chamber bydelaying the closing of the delivery valve, the trapped combustion gasesbeing subsequently used for the next fuel injection event. The lattersystem is particularly applicable in lower cost and small engineapplications where reduced complexity is often desirable.

However, the latter system presents a number of challenges in regard tocertain applications in that functionality of the fuel system may oftenbe reduced and less accurate fuel metering may be evidenced.Accordingly, an air-assisted fuel injection system which avoids the needfor separate fuel injectors and delivery valves for each combustionchamber but which maintains functionality and fuel metering accuracywould be advantageous.

To this end, the Applicant has developed a fuel injection system whicheliminates the need for a separate fuel injector for each combustionchamber. This system, which is described in the Applicant's U.S. Pat.Nos. 4,794,902, 4,841,942 and 5,024,202, uses an injection apparatushaving a delivery chamber and a delivery valve. Pressurised fuel andpressurised gas are separately supplied to the delivery chamber of theinjection apparatus. The pressure differential between the fuel and gassupplied to the chamber is regulated such that the gas pressure is lessthan the fuel pressure. During the opening of the delivery valve, anassociated valve allows the pressurised gas to flow into the deliverychamber, said pressure differential controlling the quantity of fueldelivered during the period of opening of the delivery valve.Nevertheless, whilst this fuel injection system does not require a fuelinjector for each combustion chamber, the injection apparatus utilisesnumerous components to enable the gas flow to be controlled such thatthe gas only flows when the delivery valve is opened. It is necessary toseparate the air and fuel flows until the actual injection event becauseof the difference in the gas and fuel pressures.

Another air-assisted fuel injection system is described in SAE Paper No.9,703,62 M Nuti et al, “FAST Injection System: PIAGGIO Solution for VLEV2T SI Engines”. This system incorporates a piston pump driven by acrankshaft. An air/fuel mixture is provided by a carburettor to thecrankcase of the pump. This mixture is then pressurised and transferredto a working chamber. A poppet valve delivers this mixture to thecombustion chamber of the engine, the opening pressure of the poppetvalve being regulated by a calibrated preloaded spring. Apart from thecomplexity of this arrangement, another disadvantage of this purelymechanical system is that it is not possible to control the opening andclosing times and the period of opening of the poppet valve thuslimiting the overall functionality of the system. This limits theapplicability of such a system to small engines which can toleraterelatively inaccurate control of the fuelling rate.

It is therefore an object of the present invention to provide a fuelinjector assembly, which avoids one or more of the disadvantagesreferred to above.

With this in mind, according to one aspect of the present invention,there is provided a fuel injector assembly for an internal combustionengine, including:

a delivery chamber located within the injector assembly;

a mass flow rate control means for controlling the mass flow rate offuel and compressed gas supplied to the delivery chamber, the mass flowrate being a function of a differential pressure across the mass flowrate control means; and

valve means for selectively communicating the delivery chamber to theengine to deliver fuel to the engine;

wherein the assembly is adapted such that in use, when the valve meansis opened, at least compressed gas is caused to flow thereby generatinga differential pressure across the mass flow rate control means suchthat a controlled fuel flow is provided to the engine.

According to another aspect of the present invention, there is provideda fuel injector system for an internal combustion engine, including;

at least one fuel injector assembly having a delivery chamber locatedtherein;

a fuel supply means for supplying fuel to the delivery chamber;

a compressed gas supply means for supplying compressed gas to thedelivery chamber;

a mass flow rate control means for controlling the mass flow rate of thefuel and the compressed gas supplied to the delivery chamber, the massflow rate being a function of the differential pressure across the massflow rate control means; and

valve means for selectively communicating the delivery chamber of thefuel injector assembly to the engine to deliver fuel to the engine,

wherein said system is adapted such that in use, when the valve means isopened, at least compressed gas is caused to flow thereby generating adifferential pressure across the mass flow rate control means such thata controlled fuel flow rate is provided to the engine.

The fuel and gas flow rates are a function of the differential pressureacross the mass flow rate control means. Furthermore, the amount of fueldelivered to the engine is a function of the differential pressure, thetiming of the opening of the valve means and the characteristics of themass flow rate control means.

A pressure-time fuel metering system is therefore provided. The amountof fuel may be varied by controlling the fuel and gas supply pressuresto thereby control the differential pressure. Furthermore, the amount offuel delivered to the engine may be controlled by varying the period ofopening of the valve means, and/or the start and end times of the valveperiod opening for a given gas and fuel flow rate control means anddifferential pressure.

The differential pressure is generated by the pressure loss due to theflow of the gas through the mass flow rate control means. Thisdifferential pressure then promotes fuel flow through the mass flow ratecontrol means. The differential pressure across the mass flow ratecontrol means may be the difference between the supply pressure of thesupplied fuel and the supplied compressed gas and the pressureimmediately downstream of the mass flow rate control means. Inapplications where the fuel injection system is used in direct injectionapplications, the delivery chamber pressure is affected by cylinderpressure but controlled by losses across the valve means.

The fuel injection system may further include pressure equalising meansfor at least substantially equalising the supply pressure of the fuelsupplied by the fuel supply means and the compressed gas supplied by thecompressed gas supply means to the delivery chamber.

The mass flow rate control means may include a fuel mass flow ratecontrol means and a gas mass flow rate control means. According to onepreferred embodiment, the fuel flow rate control means may be in theform of a fuel orifice and the gas flow rate control means may be in theform of a gas orifice. The fuel orifice may be located in the fuelsupply means for controlling the mass flow rate of the fuel supplied tothe delivery chamber. The gas orifice may be located in the gas supplymeans for controlling the mass flow rate of the compressed gas suppliedto the delivery chamber which instigates the differential pressure. Moreparticularly, the gas orifice may separate a gas supply passage of thefuel injector assembly from the delivery chamber, and the fuel orificemay separate a fuel supply passage of the fuel injector assembly fromthe delivery chamber.

According to another preferred embodiment, the mass flow rate controlmeans may include a venturi passage provided within the gas supplymeans. More particularly, the venturi passage may separate a gas supplypassage of the fuel injection assembly from the delivery chamber. Theventuri passage may include a throat section, and a fuel orifice may beprovided within the throat section of the venturi. The fuel orifice maybe in communication with the fuel supply means. More particularly, thefuel orifice may separate a fuel supply passage of the fuel injectorassembly from the delivery chamber. Any flow of gas through the venturipassage will instigate a differential pressure which results inentrainment of the fuel from the fuel supply means.

The gas mass flow rate for the fuel injection system may be selectedsuch that it contributes to the optimisation of the penetration rate fordifferent engine cylinder capacities (or stroke). Furthermore, thecharacteristics of the gas flow rate control means may be selected tovary the magnitude of the differential pressure. The magnitude can alsobe controlled by retarding or advancing the timing of opening of thevalve means. Therefore, advanced timings would result in a higherdifferential pressure and, conversely, at retarded timings, thedifferential pressure would be lower.

The selection of the characteristics of the fuel flow rate control meansin combination with the differential pressure determines the meteringrate of the fuel injection system. The available metering window isselected to balance the minimum and maximum engine fuelling requirementswithin the constraints of fuel containment, mixture preparation an soon.

The combined effect of the two orifii or the venturi arrangement asdescribed above establishes the average injected air-fuel ratio of themixture which will be delivered to the engine. The ratio of the orifiisize may therefore determine the air-fuel ratio of the mixture, wherebythe air-fuel ratio is typically much richer than what is necessary forcombustion.

According to one preferred arrangement, the valve means may be providedin the form of a solenoid actuated injector. The injector is located tothereby provide direct injection of fuel and compressed gas into thecombustion chamber of the engine and may be actuated by an electroniccontrol unit as a function of engine operating parameters. According toan alternative preferred embodiment, the valve means may be provided inthe form of a mechanically actuated valve located to provide for directsupply of fuel and compressed gas to the combustion chamber of theengine. Such a valve may include mechanical actuation means for openingthe valve, the duration of the valve opening being controlled as afunction of engine demand, for example by a mechanical governor foraltering the duration.

Alternatively or in addition, the valve may include spring regulationmeans, the valve opening when the pressure of the supplied fuel and gasto the valve is at or above a preset pressure. In the latterarrangement, a further valve may be required to regulate the supply offuel and/or compressed gas to the mechanical valve. This further valvemay be located on the fuel or gas supply means or immediately upstreamof the mechanical valve. The further valve may be controlled by anelectronic control unit as a function of engine operating parameters.

When the valve means is closed, the differential pressure across themass flow rate control means may be at least substantially zero andthere may therefore be no flow of fuel or compressed gas in the fuelinjection system. When the valve is opened, the gas and possibly alsothe fuel begins flowing. The gas flow generates a differential pressure,which is equal to the difference in the supply pressure of the fuel andcompressed gas and the pressure immediately downstream of the mass flowrate control means. This differential pressure is experienced across themass flow rate control means resulting in the flow of the fuel andcompressed gas to the valve means. The mass flow rate of the fuel andcompressed gas is hence a function of the abovenoted differentialpressure.

The fuel supply to the delivery chamber may be such that fuel maycontinue to be supplied to the delivery chamber for a short periodimmediately following the closing of the valve means. This may beattained by the inertia of the fuel within a fuel supply line connectedto the valve means immediately following closing of the valve means. Asdiscussed further hereinafter, the fuel flux of the fuel injectorassembly may therefore be such that the supply rate of fuel at theinitial opening of the valve means is significantly higher immediatelyfollowing the opening of the valve means. This can lead to improvedcombustion control within the engine for certain operating conditions.

The pressure equalising means may be in the form of a closed tanklocated upstream of the mass flow rate control means. A fuel supplyarrangement may supply fuel to the closed tank and a compressed gassupply arrangement may supply compressed gas to the closed tank. A floatvalve arrangement may be provided within the tank to allow fuel into thetank until the fuel reaches a preset level therein to thereby regulatethe level of the fuel within the tank. Additional fuel may then beprevented from entering the tank until the fuel level has fallen apredetermined amount. This arrangement results in at least substantialequalisation of the fuel supply pressure and the compressed gas supplypressure upstream of the mass flow rate control means. The fuel levelmay alternatively be controlled by an electronic sensor, or using an ECUstrategy for the benefit of minimising the operating time of the fuelpressure supply device. It is however also possible for a conventionalregulator or an electronic regulator to be used to equalise the fuel andgas supply pressures.

Conveniently, the absolute pressure of the pressure equalising means(ie. relative to atmospheric conditions) need not be controlled. It ispreferable to control the pressure of the gas, with the fuel pressurebeing adjusted to track the gas pressure. It is however also possiblefor the fuel pressure to be controlled, with the gas pressure beingadjusted accordingly, or to have both the fuel and gas pressuresseparately controlled.

When the pressure equalising means incorporates a tank, any fuel vapourgenerated therein by the heating of the fuel or due to the supply deviceor operating environment can also be delivered to the engine by way ofthe fuel or gas mass flow control means. A temperature sensor providedin the gas volume, combined with knowledge of the pressure within thepressure equalising means may be used to compute the fuel quantity inthe gas thus allowing correction of the duration of the fuel meteringtime.

The compressed gas may be compressed air, and the compressed gas supplymeans may include an air compressor. A pressure regulator may optionallybe provided downstream of the air compressor.

The fuel supply means may include a fuel tank and a fuel pumpoperatively arranged with respect to the fuel tank. For example, thefuel pump may be located downstream of the fuel tank.

Damper means may optionally be provided for the fuel supply line and/ora gas supply line to minimise pressure pulses within these lines.

The fuel injection system may also include check valve means forcontrolling the fuel and/or gas flows within the system. The check valvemeans can be located either upstream or downstream of the fuel flow ratecontrol means, and/or either upstream or downstream of the gas flow ratecontrol means.

The provision of a check valve means adjacent the fuel flow rate controlmeans can lead to certain benefits some of which are listed below:

it prevents the creep of air, due to buoyancy or other mechanisms, intothe fuel circuit upstream of the fuel flow rate control means. Thisenables the fuel injection system to accommodate small variationsbetween the fuel and gas supply pressures.

it contributes to the control of the time delay between the initiationof the differential pressure and the onset of fuel metering. This canlead to significant improvements in the accuracy of the fuel metering.

it de-sensitises the system to variations in “fuel head” betweencylinders on an installation which has a vertical crank axis andvertically displaced cylinders. Such an arrangement is common onoutboard marine engines.

it de-sensitises the system to pressure fluctuations which may bepresent in either the gas or fuel supply circuits.

it improves the turn-off response of the fuel metering event byproviding a more rapid drop in the differential pressure.

it de-sensitises the system to engine vibrations.

The provision of the check valve means adjacent the gas flow ratecontrol means can contribute to improved control of the time delaybetween the initiation of the pressure decay within the delivery chamberand the onset of the gas flow process.

It is also envisaged that the gas flow rate control means may beparallelled with a secondary gas flow path checked in the opposing flowdirection. The purpose of this arrangement is to have one gas flowcharacteristic when the flow-direction is from the gas supply to thecombustion chamber, and to have a typically less restrictive second gasflow characteristic when flowing gas may flow from the combustionchamber in to the gas supply circuit. In this mode of operation, gasfrom the combustion chamber is captured and used for the next injectionevent. This differential in flow rates allows the time of exposure inthis operational mode to be reduced in addition to minimising the gasquantity injected.

The characteristics of the fuel flow rate control means can be optimisedto control the phasing of the fuel supply event relative to the valveopening event to thereby provide fuel flux control. For example, it isoften preferable to provide a fuelling profile having a rich leadingedge and a lean trailing edge. The delay of the onset of fuel meteringcan be influenced by the check valve means as discussed above. Thisdelay may also be influenced by the capacitive effect of the volume ofthe delivery chamber. The larger the volume, the slower the pressuredecay rate. This leads to the slower onset of fuel metering. The delayof the end of the fuel supply event may be controlled by delaying thesubstantial equalisation of the gas and fuel pressures thereby biasingthe quantity of fuel metered after the closure of the valve means suchthat the delivery chamber can be utilised as a holding chamber. A timebased delay may also be introduced by setting or controlling thedistance, or transportation rate between the fuel flow rate controlmeans and the gas flow rate control means. Alternatively, the distancebetween the fuel and gas flow rate control means and the valve means canbe varied.

According to a further aspect of the present invention, there isprovided a method of metering fuel to an internal combustion enginehaving at least one fuel injector assembly including a delivery chamber,a mass flow rate control means for controlling the mass flow rate ofcompressed gas and the mass flow rate of fuel supplied to the deliverychamber, and valve means for selectively communicating the deliverychamber to the engine to deliver fuel to the engine,

the method including:

providing a source of fuel to the delivery chamber;

providing a source of compressed gas to the delivery chamber;

controlling the mass flow rate of fuel and gas supplied to the deliverychamber as a function of the differential pressure across the mass flowrate control means;

opening the valve means to thereby allow compressed gas to flowtherethrough resulting in a differential pressure being generated acrossthe mass flow rate control means such that a controlled fuel flow isprovided to the engine.

Conveniently, where the mass flow rate control means includes a gas flowrate control means for controlling the mass flow rate of compressed gasand a fuel flow rate control means for controlling the mass flow rate offuel, the mass flow rate of gas supplied to the delivery chamber iscontrolled as a function of the differential pressure across the gasflow rate control means and the mass flow rate of fuel supplied to thedelivery chamber is controlled as a function of the differentialpressure across the fuel flow rate control means.

The method may further include regulating the supply pressure of thefuel and of the gas to the delivery chamber such that the fuel supplypressure is at least substantially equalised with the gas supplypressure.

The delivery chamber may communicate with a combustion chamber of theengine, and hence the delivery chamber pressure is affected by cylinderpressure but is controlled by losses across the valve means.

The method may further include restricting communication of the fuelsource with the delivery chamber until the differential pressure exceedsa predetermined level. This helps to improve the accuracy of the fuelmetering to the engine for reasons that will by subsequently explained.

The amount of fuel supplied to the engine may. be controlled bycontrolling at least one of the fuel and gas supply pressures. Moreparticularly, the amount of fuel supplied to the engine may be initiallycontrolled by varying the period of opening of the valve means.Alternatively or in addition, the amount of fuel supplied to the enginemay be initially controlled by varying the start and/or end times of theopening of the valve means.

The gas supplied to the delivery chamber may be air. Other types ofgases such as an inert gas, captured combustion gases from the engine oreven LPG are however also envisaged. Furthermore, the fuel supplied tothe delivery chamber is typically in liquid form, although the supply ofgaseous fuels are also envisaged.

The fuel injection system according to the present invention henceprovides a dual fluid fuel system which retains the advantages of suchsystems, (ie, improved atomisation of the fuel and fuel sprayformation). This fuel injection system however has less components thancertain versions of the Applicant's earlier electronic fuel injectionsystems and eliminates the need for separate fuel and gas solenoidactuated injectors. In some applications, even the need for any suchsolenoid actuated injectors may be eliminated. This leads to significantcost savings and less complexity in the control of the fuel injectionsystem of the present invention whilst maintaining functionality.

It will be convenient to further describe the present invention withrespect to the accompanying drawings which illustrate preferredembodiments of the invention. Other embodiments of the invention arepossible and consequently, the particularity of the accompanyingdrawings is not to be understood as superseding the generality of thepreceding description of the invention.

In the drawings:

FIG. 1 is a schematic view of a first preferred embodiment of a fuelinjector assembly according to the present invention;

FIG. 2 is a schematic view of a second preferred embodiment of a fuelinjector assembly according the present invention;

FIG. 3 is a schematic view of a fuel injection system according to thepresent invention;

FIG. 4 is a cross-sectional view of a third preferred embodiment of afuel injector assembly according to the present invention;

FIG. 5 is a plot illustrating the operation of the fuel injectorassembly of FIG. 4; and

FIG. 6 is a graphical representation of the typical fuel flux profilesof a fuel injector assembly of the present invention in comparison withan electronic fuel metering system.

The present invention can be used to provide direct fuel injection intothe combustion chamber of an engine and, whilst not limited as such, thefuel injection system of the present invention will hereinafter bedescribed for such an application.

FIG. 1 illustrates the principle of operation of a fuel injectorassembly and fuel injection system according to the present invention.The fuel injector assembly 1 has a delivery chamber 5 therein. A fuelsupply means 2 delivers fuel via a fuel supply line 6 to the deliverychamber 5. The mass flow rate of the fuel into the delivery chamber 5 iscontrolled by means of a fuel flow rate control means 8 locateddownstream of the fuel supply line 6. The fuel flow rate control means 8is shown as a fuel orifice in FIG. 1.

Compressed gas is also supplied to the delivery chamber 5 through a gassupply line 3. The mass flow rate of the gas supplied to the deliverychamber 5 is similarly controlled by a gas flow rate control means 7located downstream of the gas supply line 3. The gas flow rate controlmeans 7 is shown as a gas orifice in FIG. 1. The fuel flow rate controlmeans 8 and the gas flow rate control means 7 together define the massflow rate control means 50 as shown in FIG. 1.

The fuel injection system regulates the fuel supply pressure P_(f) andthe gas supply pressure P_(g) such that the fuel and gas supplypressures are substantially equalised.

The delivery chamber 5 is in cyclic communication with an enginecombustion chamber (not shown). The communication of the deliverychamber 5 to the combustion chamber is controlled by a valve assembly 9schematically shown as a poppet valve in FIG. 1. The valve assembly 9may typically be a delivery or air injector such as that described inthe Applicants' aforementioned U.S. Pat. No. 4,934,329. When thedelivery chamber 5 is isolated from the combustion chamber, the fuel andgas supply pressures P_(f), and P_(g) are substantially the same as thepressure within the delivery chamber P_(i). There is thereforesubstantially no differential pressure across the fuel flow rate controlmeans 8 or across the gas flow rate control means 7. Therefore, littleto no fuel and gas flow through the respective orifii into the deliverychamber 5 when the valve 9 is closed. The provision of the respectiveoptional check valves 39 ensure that there is no flow until thedifferential pressure reaches a desired level.

Following opening of the valve assembly 9, a pressure difference isestablished between the fuel and gas supply pressures P_(f), P_(g) andthe delivery chamber pressure P_(i) which is primarily instigated by therestriction of gas flow. This results in a differential pressure beingdeveloped across both the fuel orifice 8 and the air orifice 7. Thedifferential pressure produces a fuel flow through the fuel orifice 8into delivery chamber 5 and an air flow through the air orifice 7 intothe delivery chamber 5. An air/fuel mixture can then be delivered fromthe delivery chamber 5 by way of the valve 9 to the combustion chamber.

The amount of fuel supplied to the engine is therefore a function of thedifferential pressure produced when the valve 9 is opened as well as theduration of the opening of the valve 9. The fuel injection system istherefore similar to the Applicant's aforementioned earlier fuelinjection systems in that it is based on a pressure-time deliveryprinciple. The principal difference is that the need for a separate fuelinjector for each combustion chamber is eliminated.

FIG. 2 shows another preferred embodiment of a fuel injection system andfuel injector assembly according to the present invention. It should benoted that features corresponding to those shown in FIG. 1 aredesignated with the same reference numerals for clarity reasons. Theprinciple difference with the embodiment shown in FIG. 1 is that the gassupply line 3 supplies gas to the delivery chamber 5 through a venturipassage 4. The mass flow rate of the gas supplied to the deliverychamber is controlled by the throat 7 a of the venturi 4 which operatesin the same way as the gas orifice 7 of FIG. 1. Fuel is delivered from afuel supply means 2 and through a fuel supply line 6 to a fuel orifice 8a provided at the venturi throat 7 a, the fuel orifice 8 a of the fuelsupply line 6 operating in the same way as the fuel orifice 8 of FIG. 1.This embodiment otherwise operates in the same way as the embodiment ofFIG. 1, with the throat 7 a and the fuel orifice 8 a together definingthe mass flow rate control means 50.

FIG. 3 provides an overall fuel injection system schematic showing onepreferred embodiment of this system for an engine 10. The compressed gasis supplied by a compressor 12 which delivers compressed gas through agas passage 13 to an air duct 15 of an air and fuel rail 11 of theengine 10. The air duct 15 provides the compressed gas to the or eachdelivery or fuel injector assembly 1, an injector assembly 1 beingprovided for each cylinder of the engine 10. The gas pressure within theair duct 15 is further regulated by a regulator 14 in communication withthe air duct 15.

Part of the compressed gas is diverted through a bypass line 13 a to apressure equalising means 19. This pressure equalising means 19 is inthe form of a tank 20 containing a float valve 21 therein. Fuel issupplied from a fuel tank (not shown) through a fuel passage 17 to thepressure equalising means 19. The fuel is delivered to the tank 20 ofthe pressure equalising means 19 using a high pressure fuel pump 18. Alift pump 16 may also be provided upstream of the fuel pump 18 whererequired. The fuel supply to the tank 20 is controlled by the floatvalve 21. Fuel is allowed to flow through fuel supply passage 17 a intothe tank 20 until the fuel level within the tank 20 reaches apre-determined point, at which time the float valve 21 closes to preventfurther fuel flow into the tank 20. Excess fuel is then redirected to afuel bypass line 17 b back to the fuel supply passage 17. The one wayvalve 17 c on the fuel bypass line 17 b acts as a limiter to preventover pressurisation within the system upstream of the fuel pump 3.

Because compressed gas is also provided to the tank 20, this results insubstantial equalisation of both the fuel and gas pressures therein.Fuel from the tank 20 is then provided through a further fuel supplypassage 22 to a fuel duct 23 of the air and fuel rail 11 of the engine10. The fuel duct 23 then supplies fuel to the injectors 1. Any fuelvapour which may have accumulated within the tank 20 of the pressureequalising means 19 can also be delivered through to the fuel duct 23for subsequent combustion by the engine 10 by way of a fuel vapour line24. The injectors 1 control the flow rate of the fuel to the engine 10on the basis of the differential pressure created across the mass flowrate control means 50 in the manner as described previously.

FIG. 4 shows a further preferred embodiment of a fuel injector assembly1 according to the present invention. The same reference numerals tothose used for corresponding components in FIGS. 1, 2 and 3 are used inFIG. 4 for clarity purposes. FIG. 4 shows an injector 1 according to thepresent invention supported on the air and fuel rail 11. The valveassembly 9 is shown as a solenoid actuated injector having an injectornozzle 35, the end of which is located within an engine combustionchamber (not shown). A poppet valve 36 controls the flow of the air/fuelmixture into the combustion chamber. The movement of the poppet valve 36is actuated by the cyclic energisation of a solenoid coil 37 in theknown manner, with an armature 38 actuated by the solenoid coil 37 beingoperatively connected to the poppet valve 36.

A housing 28 is provided upstream of the valve assembly 9 and is locatedwithin a cavity 29 within the air and fuel rail 11. The housing 28accommodates the delivery chamber 5. Air is delivered through the airduct 15 of the air and fuel rail 11, with a passage 27 being providedfrom the air duct 15 to an air orifice 7 located within a side wall ofthe housing 28 to the delivery chamber 5. Fuel is supplied through thefuel duct 23 to a fuel cavity 33 within the housing 28. A fuel screen 34filters the fuel prior to passing through a fuel orifice disc 30providing the fuel orifice 8. The mass flow rate control means 50 isprovided by the elements shown in the confines of the dotted lines inFIG. 4.

A check valve assembly 31 is provided downstream of the fuel orificedisc 30. The purpose of the check valve assembly 31 is to preventcompressed gas from seeping into the fuel supply where there is anyvariation between the gas and fuel supply pressures. The check valveassembly 31 however also leads to operational advantages as best shownby referring to FIG. 5.

In operation the initial opening of the valve assembly 9 results in thestart of a flow of gas through the air orifice 7 from the air duct 15.Some fuel flow may also occur across the fuel orifice 8 at this time.However the provision of a check valve 39 in the fuel line as alluded tohereinbefore prohibits this until the differential pressure reaches adesired level. The gas flow generates a differential pressure which thenproduces a controlled fuel flow of fuel.

The fuel injector assembly 1 operates in the manner previouslydescribed. To reiterate, when the delivery chamber 5 is isolated fromthe combustion chamber, the fuel and gas supply pressures P_(f), andP_(g)are substantially the same as the pressure within the deliverychamber P_(i). There is therefore substantially no differential pressureacross the fuel flow rate control means 8 or across the gas flow ratecontrol means 7. Therefore, little to no fuel and gas flow through therespective orifii into the delivery chamber 5 when the valve 9 isclosed. The provision of the respective optional check valves 39 ensurethat there is no flow until the differential pressure reaches a desiredlevel.

Following opening of the valve assembly 9, a pressure difference isestablished between the fuel and gas supply pressures P_(f), P_(g) andthe delivery chamber pressure P_(i) which is primarily instigated by therestriction of gas flow. This results in a differential pressure beingdeveloped across both the fuel orifice 8 and the air orifice 7. Thedifferential pressure produces a fuel flow through the fuel orifice 8into delivery chamber 5 and an air flow through the air orifice 7 intothe delivery chamber 5. An air/fuel mixture can then be delivered fromthe delivery chamber 5 by way of the valve 9 to the combustion chamber.

FIG. 5 shows a series of plots, the top plot V showing the voltagesignal to the solenoid coil 37, the middle plot S showing thedisplacement of the poppet valve 36 as a result of the voltage signaland the lower most plot P showing the change in the internal pressurewithin the delivery chamber 5 due to the opening of the poppet valve 36.From the plots it can be noted that following a short delay after thevoltage signal is received by the injector assembly 9, the poppet valve36 opens to its fully open position as shown at level 0 of plot S. Asthe poppet valve 36 opens, the differential pressure across the fuel andair orifii progressively increases until it reaches a steady state valueas shown at level C on plot P. The check valve assembly 31 normallyprevents fuel from entering the delivery chamber 5 until thedifferential pressure reaches a pre-set level as shown at level A onplot P. This prevents fuel seeping into. the delivery-chamber 5 as aresult of fluctuations of the internal pressure therein, for example dueto variations in the gas supply pressure. Furthermore, the check valveassembly 31 only allows for delivery of fuel through the fuel orifice 8when the differential pressure exceeds the pre-set level. This acts toprovide for more accurate fuel metering by the fuel injection system.

Following closure of the poppet valve 36, there is a small delay beforethe internal pressure within the delivery chamber 5 returns to itsinitial level. This results in the continuing supply of fuel to thedelivery chamber 5 for a short period after closure of the poppet valve36. A small supply of fuel is then held within the delivery chamber 5until the next injection event. This leads to a beneficial fuel fluxfrom the fuel injector because a rich air fuel mixture is initiallysupplied when the injector 1 is opened, the air fuel mixture becomingprogressively leaner near the end of the injection period.

FIG. 6 show typical examples of the fuel flux profiles achievable withthe fuel system of the current invention compared with an electronicfuel metering system such as that described in the Applicants' U.S. Pat.No. 4,934,329. It is to be understood that optimisation of the physicalgeometry of the passive fuel metering system of the current inventionallows for the air-fuel interaction to be controlled to thereby allowfor the similar variation of the fuel metering profile as is achievablewith electronic fuel phasing control. For example, it is found that bychanging the distance between the fuel and gas flow rate control meansand the valve means, the delivery profiles achieved by the electronicsystem could be approximated. As can be seen in FIG. 6, the typicaldelivery profiles of the current invention compare favourably with thedelivery profiles of the systems utilising electronic fuel metering.

It is accepted that in fuel injection systems, a fuel rich leading edgehelps attain higher performance due to the longer time available forin-cylinder air utilisation. Further to this, it is also accepted that aleaner trailing edge assists in enhancing ignitability and allows formore retarded injection windows to be used. As is evident from FIG. 6,the fuel flux profiles of both these systems show a higher degree ofmodulation than the typical square wave of a single fluid system.

The fuel injection system according to the present invention leads to anumber of benefits over and above known dual fluid injection systems. Noextra fuel injector is needed as would be required in the Applicants'earlier fuel injection system.. The fuel injection system howeverretains functionality while at the same time being a relatively simplersystem.

The above description is provided for the purposes of exemplificationonly and it will be understood by a person skilled in the art thatmodifications and variations may be made without departing from theinvention.

What is claimed is:
 1. A fuel injection assembly for an internalcombustion engine comprising: a delivery chamber located within the fuelinjector assembly; a gas supply means for supplying gas; a mass flowrate control means for controlling the mass flow rate of fuel and gassupplied to the delivery chamber, the mass flow rate being a function ofa differential pressure across the mass flow rate control means, whereinthe mass flow rate control means includes at least a gas mass flow ratecontrol means, the gas mass flow rate control means being a gas orificelocated in the gas supply means; and valve means for selectivelycommunicating the delivery chamber to the engine to deliver fuel to theengine, wherein when the valve means is opened, at least compressed gasis caused to flow thereby generating a differential pressure across themass flow rate control means such that a controlled fuel flow isprovided to the engine.
 2. A fuel injector assembly according to claim1, wherein the mass flow rate control means includes a fuel mass flowrate control means and the gas mass flow rate control means.
 3. A fuelinjector assembly according to claim 2, wherein the fuel mass flow ratecontrol means is a fuel orifice located in a fuel supply means.
 4. Afuel injector assembly according to claim 3, wherein the gas orificeseparates a gas supply passage of the fuel injector assembly from thedelivery chamber, and the fuel orifice separates a fuel supply passageof the fuel injector assembly from the delivery chamber.
 5. A fuelinjector assembly according to claim 2, wherein the mass flow ratecontrol means includes a venturi passage having a throat,.the venturipassage being located in the gas supply means and a fuel orificeprovided in the throat section of the venturi passage, the fuel orificebeing in communication with a fuel supply means.
 6. A fuel injectorassembly according to claim 5, wherein the venturi passage separates agas supply passage of the fuel injector assembly from the deliverychamber, and the fuel orifice separates a fuel supply passage of thefuel injector assembly from the delivery chamber.
 7. A fuel injectorassembly according to claim 1, further including a check valve locatedupstream and adjacent the fuel mass flow rate control means.
 8. A fuelinjector assembly according to claim 1, further including a check valvelocated downstream and adjacent to fuel mass flow rate control means. 9.A fuel injector assembly according to claim 1, further including a checkvalve located upstream and adjacent the gas mass flow rate controlmeans.
 10. A fuel injector assembly according to claim 1, furtherincluding a check valve located downstream and adjacent the gas massflow rate control means.
 11. A fuel injector assembly according to claim1, wherein the valve means includes a solenoid actuated poppet valve.12. A fuel injector assembly according to claim 1, valve means includesa mechanically actuated poppet valve.
 13. A fuel injector assemblyaccording to claim 12, wherein the valve means includes springregulation means for preventing the opening of the valve means until thegas and/or fuel supply pressure to the valve means is above apredetermined level.
 14. A fuel injection system for an internalcombustion engine including an air and fuel rail for supporting at leastone fuel injector assembly according to any one of the preceding claims,the air and fuel rail having an air duct and a fuel duct for enablingcompressed gas and fuel to be respectively supplied to the at least onefuel injector assembly.
 15. A fuel injection system for an internalcombustion engine, comprising: at least one fuel injector assemblyhaving a delivery chamber located therein; a fuel supply means forsupplying fuel to the delivery chamber; a gas supply means for supplyingcompressed gas to the delivery chamber; a mass flow rate control meansfor controlling the mass flow rate of the fuel and the compressed gassupplied to the delivery chamber, the mass flow rate being a function ofthe differential pressure across the mass flow rate control means,wherein the mass flow rate control means includes at least a gas massflow rate control means, the gas mass flow rate control means being agas orifice located in the gas supply means; and valve means forselectively communicating the delivery chamber of the fuel injectorassembly to the engine to deliver fuel to the engine, wherein when thevalve means is opened, at least compressed gas is caused to flow therebygenerating a differential pressure across the mass flow rate controlmeans such that a controlled fuel flow rate is provided to the engine.16. A fuel injection system according to claim 15, wherein the mass flowrate control means includes a fuel mass flow rate control means and thegas mass flow rate control means.
 17. A fuel injection system accordingto claim 16, wherein the fuel mass flow rate control means is a fuelorifice located in a fuel supply means.
 18. A fuel injection systemaccording to claim 17, wherein the gas orifice separates a gas supplypassage of the fuel injector assembly from the delivery chamber, and thefuel orifice separates a fuel supply passage of the fuel injectorassembly from the delivery chamber.
 19. A fuel injection systemaccording to claim 16, wherein the mass flow rate control means includesa venturi passage having a throat section, the venturi passage beinglocated in the gas supply means and a fuel orifice provided in thethroat section of the venturi passage, and fuel orifice being incommunication with the fuel supply means.
 20. A fuel injection systemaccording to claim 19, wherein the venturi passage separates a gassupply passage of the fuel injector assembly from the delivery chamber,and the fuel orifice separates a fuel supply passage of the fuelinjector assembly from the delivery chamber.
 21. A fuel injection systemaccording to any one of claims 15 to 20, further including a check valvelocated upstream and adjacent the fuel mass flow rate control means. 22.A fuel injection system according to any one of claims 15 to 20, furtherincluding a check valve located downstream and adjacent the fuel massflow rate control means.
 23. A fuel injection system according to anyone of claims 15 to 20, further including a check valve located upstreamand adjacent the gas mass flow rate control means.
 24. A fuel injectionsystem according to a claim 15, further including a check valve locateddownstream and adjacent the gas mass flow rate control means.
 25. A fuelinjection system according to claim 15, wherein the valve means includesa solenoid actuated poppet valve.
 26. A fuel injection system accordingto claim 15, wherein the valve means includes a mechanically actuatedpoppet valve.
 27. A fuel injection system according to claim 15, whereinthe valve means includes spring regulation means for preventing theopening of the valve means until the gas and/or fuel supply pressure tothe valve means is above a predetermined level.
 28. A fuel injectionsystem according to claim 15, further including a pressure equalisingmeans for at least substantially equalising the supply pressure of thefuel and the compressed gas supplied to the at least one fuel injectorassembly.
 29. A fuel injection system according to claim 28, wherein thepressure equalising means includes a closed tank to which compressed gasand fuel is supplied and a fuel control means for controlling the amountof fuel supplied to the tank.
 30. A fuel injection system according toclaim 29, wherein the fuel control means is a float valve adapted toprevent further fuel supply to the tank when the fuel reaches apredetermined level therein.
 31. A fuel injection system according toclaim 29, wherein the fuel control means is a fuel level switch.
 32. Amethod of metering fuel to an internal combustion engine comprising atleast one fuel injector assembly including a delivery chamber, a massflow rate control means for controlling the mass flow rate of compressedgas and the mass flow rate of fuel supplied to the delivery chamber, andvalve means for selectively communicating the delivery chamber to theengine to deliver fuel to the engine, the method comprising the stepsof: providing a source of fuel to the delivery chamber; providing asource of compressed gas to the delivery chamber; controlling the massflow rate of fuel and gas supplied to the delivery chamber as a functionof the differential pressure across the mass flow rate control means, byopening the valve means to cause compressed gas to flow therethroughresulting in a differential pressure across the mass flow rate controlmeans to result in a controlled fuel flow to the engine, wherein themass flow rate control means includes at least a mass flow rate controlmeans, the gas mass flow rate control means being a gas orifice locatedin a gas supply means.
 33. A method according to claim 32, wherein themass flow rate control means includes a gas flow rate control means forcontrolling the mass flow rate of compressed gas and a fuel flow ratecontrol means for controlling the mass flow rate of fuel, the mass flowrate of gas supplied to the delivery chamber being controlled as afunction of the differential pressure across the gas flow rate controlmeans and the mass flow rate of fuel supplied to the delivery chamberbeing controlled as a function of the differential pressure across thefuel flow rate control means.
 34. A method according to claim 32,further including regulating the supply pressure of the fuel and of thegas to the delivery chamber such that the fuel supply pressure is atleast substantially equalised with the gas supply pressure.
 35. A methodaccording to claims 32, further including restricting the communicationof the source of fuel to the delivery chamber until the differentialpressure exceeds a predetermined level.
 36. A method according to claims32, including controlling the amount of fuel supplied to the engine bycontrolling at least one of the fuel and gas supply pressures.
 37. Amethod according to claims 32, including initially controlling theamount of fuel supplied to the engine by varying the period of openingof the valve means.
 38. A method according to claims 32, includinginitially controlling the amount of fuel supplied to the engine byvarying the start and/or end times of the opening of the valve means.39. A fuel injector assembly for an internal combustion engine,comprising: a delivery chamber located within the fuel injectorassembly; a mass flow rate control means for controlling a mass flowrate of fuel and compressed gas supplied to the delivery chamber, themass flow rate being a function of a differential pressure across themass flow rate control means, wherein the mass flow rate control meansincludes a venturi passage having a throat section, the venturi passagebeing located in a gas supply means and a fuel orifice provided in thethroat section of the venturi passage, the fuel orifice being incommunication with a fuel supply means; and valve means for selectivelycommunicating the delivery chamber to the engine to deliver fuel to theengine, wherein when the valve means is opened, at least compressed gasis caused to flow thereby generating a differential pressure across themass flow rate control means such that a controlled fuel flow isprovided to the engine.
 40. A fuel injection system for an internalcombustion engine, comprising: at least one fuel injector assemblyhaving a delivery chamber located therein; a fuel supply means forsupplying fuel to the delivery chamber; a compressed gas supply meansfor supplying compressed gas to the delivery chamber; a mass flow ratecontrol means for controlling a mass flow rate of the fuel andcompressed gas supplied to the delivery chamber, the mass flow ratebeing a function of a differential pressure across the mass flow ratecontrol means, wherein the mass flow rate control means includes aventuri passage having a throat section, the venturi passage beinglocated in the gas supply means and a fuel orifice provided in thethroat section of the venturi passage, the fuel orifice being incommunication with the fuel supply means; and valve means forselectively communicating the delivery chamber of the fuel injectorassembly to the engine to deliver fuel to the engine, wherein when thevalve means is opened, at least compressed gas is caused to flow therebygenerating a differential pressure across the mass flow rate controlmeans such that a controlled fuel flow rate is provided to the engine.